A control object is driven and operated by a PWM (i.e., pulse width modulation) driver so that a control amount such as a voltage or a current is controlled to be a target amount. Thus, a digital feedback control operation is performed. In this digital feedback control operation, a sampling data as a control object includes a PWM noise, which has a fundamental wave component with a PWM driving frequency and a harmonic component. In this case, the PWM noise is removed using an analog filter, which has a cut-off frequency lower than a Nyquist frequency, is arranged before an A/D converter or a sampling and holding circuit so that an aliasing in a sampling operation is restricted (JP 2003-46390 A corresponding to US 2003/0025625 A1). When the aliasing occurs, a PWM noise component, which is folded back, overlaps in a signal frequency range of the sampling data. Thus, the feedback operation becomes unstable.
However, when a response frequency of the control object is nearly equal to the PWM driving frequency, it is necessary to arrange an analog filter having a steep attenuation property with a high order. This analog filter is provided by a large circuit, and therefore, a variation of a filtering property caused by a variation of a constant value may be generated. Alternatively, the sampling frequency is increased using the A/D converter having a short conversion time, and the sampling data is input with restricting the aliasing. Further, the sampling data is processed by the digital filter so that the PWM noise is removed. However, in this case, it is necessary to prepare the A/D converter having a high conversion speed and high performance and the digital filter having the high order and the steep attenuation property with a complicated construction.